Magnetic nanoparticles (MNs) are interesting because the magnetic properties they display are much different than that of the bulk material. MNs have a great potential to be used in applications such as magnetic ferrofluids, contrast agents for imaging, biomedical applications, and drug delivery. However, due to MN dipole-dipole attractions, MNs tend to aggregate. To help prevent aggregation, polymeric stabilizers are usually grafted onto the surface of the MNs. Hindering particle aggregation is important to preserve the magnetic properties of the particles as well as permitting a good dispersion of the particles in be obtained.
Polymers can be anchored to the surface of nanoparticles by: physisorption, where a weak bond is formed between the particle surface and the polymer; grating to technique, wherein a polymer end-group reacts with the particle surface and grafting from technique, which involves the growth of polymer chains from one end of the chain initiator anchored to the particle surface through chemisorption.
Out of these three methods, the grafting from technique gives the highest grafting density because polymer chains are grown from small molecules reacting with the surface of the MN and the tethered polymer chains are forced to stretch away from the surface. In using the “grafting from” method, macro-initiators (MIs) are produced for the grafting of initiators onto the surface of the MNs. Atom transfer radical polymerization (ATRP) initiators are commonly used because narrow polydispersed polymers can be obtained with relatively low radical concentration. Anchoring moieties that have been used to graft ATRP initiators to magnetic nanoparticles include, but are not limited to: phosphonate (—PH(OH)2), chlorosilane (—SiCl3), carboxylic acid group (COOH). In addition, peroxide based and azo compounds have been grafted onto MNs in order to create a polymer shell via the polymerization of vinyl monomers.
In the production of magnetic macro-initiators, magnetite has almost exclusively been used as the magnetic core, limiting the magnetic properties that can be achieved by use of different magnetic cores. In addition, the synthesis of magnetic MIs has mainly focused on the production of a polymer shell surrounding the MI. Thus, there is a need in the art to produce magnetic macro-initiators that possess different magnetic properties such that the particles can be matched to an end application. Furthermore, the art would benefit from the production of stable magnetic MIs that are capable of forming networks within a polymeric system during polymerization thereby imparting magnetic properties into non-magnetic polymer materials.
The magnetic initiators disclosed herein will be useful in the generation of chain reactions, including particularly the formation of polymers and crosslinking of systems through the generation of free radicals. The most important part in free radical polymerization is activation of the initiator, i.e., the formation of a free radical. Initiation is most commonly achieved by thermal or radiation curing. Thermal curing can be accomplished by directly cleaving a bond at high temperatures or at lower temperature by use of thermal initiators. Whether or not thermal curing can be employed is highly dependent on the heat sensitivity of the substrate. Radiation curing has the advantage of a close to ambient temperature cure and can be accomplished by means of many different radiation sources, the most prominent being ultraviolet (IN) and electron beam (EB). In UV curing, photoinitiators are decomposed into free radicals which initiate the polymerization process, while in EB curing, monomers are directly excited by the high energy beam. Both UV and EB curing can be used on a much broader selection of substrates as opposed to thermal curing. However, these are not without their drawbacks. UV curing cannot be performed on heavily pigmented films because the pigments contained within the film can absorb or scatter the incoming radiation, causing the intensity of the UV rays propagating through the film to continually decease with depth. However, the decrease in radiation intensity due to pigment is not experienced with EB curing. Even so, this advantage comes with a higher safety concern which requires extensive safety equipment and measures. Although UV and EB curing may have different radical producing mechanisms, each shares one major flaw: in order for proper and uniform curing to take place, radiation needs to be distributed evenly throughout material. Consequently, these methods are usually limited to parts that can be easily manipulated around a radiation source allowing for uniform coverage.
Thus the art would further benefit from a new initiation method achieving chain reactions without the use of heat or application or radiation.
Previous work on magnetically cured systems have all focused on the generation of thermal heat throughout the bulk of an adhesive or composite via vibration of magnetic nanoparticles contained in the material. Heat is generated internally by means of vibrating magnetic particles embedded in the adhesive. The vibrations arise due to the application of an AC MF. When the field is on, the magnetic particles align themselves parallel to the applied MF and then reorganize into a disordered state once the field is switched off. The vibrations of the magnetic particles allow heat to be built up using a high frequency. The drawback of this curing method is that the large amount of generated heat precludes curing on heat sensitive substrates. Distinct from these methods, the method of the present invention provides for the initiation of free radicals via the vibration of magnet MIs without the generation of deleterious heat.